The present invention relates to a device for transmitting rotary motion to a plate-holding impression cylinder of a plate mounting machine for flexographic printing, suitable to mount the printing plates and to print proofs with the cylinders thus prepared.
Machines such as the one disclosed in U.S. Pat. No. 5,666,881 by the same Applicant are available for mounting printing plates and for printing proofs. These machines have, at the front and in an upward region, a backup roll which is mounted horizontally and so that it can rotate between the sides of the machine, and have, at the front and in a downward region, a horizontal beam which performs a vertical translational motion and has crescent-shaped rests which rotatably support end pivots of an impression cylinder.
The backup roll is covered with a sheet of paper, on which reference markings are provided along generatrices (straight lines) and along directrices (circular lines) of the roller by means of a writing stylus with which said machines are provided for this purpose.
The impression cylinder is covered with an appropriate mounting jacket on which double-adhesive material is used to fix one or more printing plates when the impression cylinder, supported by the beam, is in a mounting position which is spaced from the backup roll.
Optical means are used to precisely position the printing plates on the impression cylinder and are constituted by a semitransparent mirror, which runs along the entire length of the machine, and by video cameras and monitors. Said optical means assist in the alignment of dots and markings provided on the backup roll with dots seen through the mirror on the printing plate.
In printing proofs, the backup roll and the impression cylinder are arranged peripherally adjacent and are actuated so that their peripheral speeds are synchronous. The cylinders can be actuated by means of a gearmotor coupled to a gear system for the kinematic connection of the roller and the cylinder. Said gear system is constituted by a driving gear, which is keyed to the shaft of the gearmotor, and by a first gear, which is coaxial to the axis of the impression cylinder and meshes with a second gear, which is coaxial to the axis of the backup roll.
One problem that is frequently noted in these machines is that depending on the print format to be reproduced on the impression roller it is necessary to change not only the impression cylinder but also the first gear associated with the impression cylinder, since the first gear and the impression cylinder must have the same first pitch circle radius. This entails that for each print format it is necessary to change the corresponding first gear.
Changing the first gear for each print format considerably increases the time required to mount the mounting jacket and accordingly increases the time required to prepare the printing plates in order to then perform the necessary print proofs.
Another problem is linked to the fact that the pitch of the gears conditions the length of the print to be reproduced. The printing length is in fact strictly dependent on the number of pitches, in that the print length can never require a print whose length is equal to a certain number of pitches plus a fraction of a pitch. Furthermore, this entails a waste of material.
Moreover, the presence of gears causes very poor definition of the image to be reproduced, since the gears cause vibrations that are transmitted to the cylinders during printing.
Finally, it is necessary to store and purchase a large number of gears to be adapted to all possible printing formats. This obviously entails corresponding storage problems.
These problems have been partly solved by a type of machine in which the backup roll and the impression cylinder are actuated by respective motor means, which directly actuate the roll and the cylinder so that they have synchronized speeds in their peripheral region. Connection of the respective motor means to the impression cylinder occurs according to the most disparate criteria.
The aim of the present invention is to obviate the above mentioned drawbacks by providing a device for transmitting rotary motion to an impression cylinder.
Within this aim, an object of the present invention is to provide a connection device in which it is possible to insert/extract the mounting jackets with the aid of devices that support the impression cylinder in a cantilevered fashion and of pneumatic devices that do not occupy considerable space on the machine.
Another object of the present invention is to be able to mount, on the same machine, any kind of impression cylinder, including old-generation cylinders.
Still another object of the present invention is to achieve said aim with a structure which is simple, relatively easy to provide in practice, safe in use, effective in operation, and relatively low in cost.
This aim and this and other objects which will become better apparent hereinafter are achieved by the present device for transmitting rotary motion to a plate-holding impression cylinder of a plate mounting machine for flexographic printing, which is provided at the front, in a horizontally parallel arrangement which allows them to move with respect to each other between a mounting position and a printing position, a backup roll, which can be actuated rotatably by first motor means and is meant to be covered by a sheet of paper provided with reference markings; a beam, which is provided with crescent-shaped rests meant to rotatably support end pivots of said impression cylinder, can be actuated by second motor means and is covered by a jacket for mounting said printing plate; said impression cylinder and said backup roll having synchronized peripheral speeds in said printing position; said device being characterized in that it comprises a gear which is coaxial to said impression cylinder and is rotatably supported by a head associated with said beam, said gear being actuatable by said second motor means, and a hollow transmission shaft which is coaxial to said gear and detachably engages at one end, by way of first coupling elements, one of said pivots and engages, at the other end, by way of second coupling elements, said gear; said transmission shaft, with respect to said mounting position, being movable coaxially to said gear between a retracted disengagement configuration and a protruding configuration for connecting said coupling elements to said impression cylinder and to said gear in order to rotationally couple said impression cylinder to said second motor means.